This invention relates to a process for manufacturing a semiconductor device comprising forming a thermal oxide film. In particular, it relates to a process for manufacturing a semiconductor device, whereby a high quality thermal oxide film can be formed.
A thermal oxide film has been formed with a vertical diffusion furnace 10, for example, as illustrated in FIG. 3. This vertical diffusion furnace comprises a cylindrical diffusion furnace 1 made of a quartz tube whose lower end is opened. For thermal oxidation, first, a wafer boat 3 on which a plurality of semiconductor wafers are placed is inserted from the opening and then the opening is closed. The diffusion furnace 1 is externally heated with a heater 4 surrounding the diffusion furnace 1. While maintaining the status, gases such as nitrogen N2, oxygen O2 and steam H2O are fed into the diffusion furnace 1 through a gas inlet 5 on the top of the diffusion furnace 1, to thermally oxidize the semiconductor wafers on the wafer boat 3.
Atmosphere gases such as nitrogen N2, oxygen O2 and moisture H2O are, as appropriate, fed in the diffusion furnace 1 in each of a sequence of steps including inserting the wafer boat 3 into the diffusion furnace 1; stabilizing an internal temperature of the furnace; oxidizing for forming an oxide film; annealing; and removing the wafer boat 3 from the furnace. In particular, after annealing, the wafer boat 3 is removed under an atmosphere of high purity nitrogen for avoiding further oxidation.
Such a process for forming a thermal oxide film employing the vertical diffusion furnace 10 has been, however, inadequate in terms of, for example, a stress lifetime in the resulting thermal oxide film.
Thus, we have conducted a sequence of thermal oxidation using a diffusion furnace equipped with a load-lock chamber as described in Japanese Patent Laid-Open No. 4-329630, so-called xe2x80x9ca load-lock diffusion furnacexe2x80x9d, in the light of the fact that in a process employing the conventional diffusion furnace, oxygen and other impurity gases as well as particles in the air are incorporated in atmosphere gases due to uptake of the air during inserting a semiconductor wafer into the diffusion furnace 1 made of a quartz tube or removing the semiconductor wafer from the diffusion furnace 1.
The load-lock diffusion furnace 100 comprises a load-lock chamber 6 in a lower opening in the diffusion furnace 1 made of a quartz tube as illustrated in FIG. 1. This load-lock chamber 6 further comprises a pump DP for degassing as well as a gate G1 for separating the diffusion furnace 1 from the load-lock chamber 6, a gate G2 for separating the load-lock chamber 6 from the outer atmosphere and a gas inlet 7 for feeding nitrogen gas into the load-lock chamber 6.
Using the load-lock diffusion furnace 100, thermal oxidation was conducted as follows. First, in the diffusion furnace 1, oxidation is conducted under a high purity atmosphere gas of hydrogen and oxygen. Then after replacing the atmosphere gas in the diffusion furnace 1 with high purity nitrogen, the system is cooled. Then, the pump DP is turned on to evacuate the load-lock chamber 6, nitrogen gas is fed through the gas inlet 7, the gate G1 is opened and the wafer boat 3 is transferred into the load-lock chamber 6.
As described above, a cooling and a removing steps were conducted under the atmosphere of high purity nitrogen free from impurity gases such as oxygen or particles in the air.
It was, however, found that in this process for forming a thermal oxide film, strain occurs in an SiO2/Si bond due to stress generated in an interface between a thermal oxide film and a silicon substrate, during the cooling and the removing steps, giving an insulating film susceptible to stress. Furthermore, determination of a lifetime for the thermal oxide film formed demonstrated that the lifetime was significantly reduced than that for a thermal oxide film formed by a process using a conventional vertical diffusion furnace 10 as illustrated in FIG. 3.
Thus, to solve the above continuing problems in the prior art, this invention aims at providing a process for manufacturing a semiconductor device, which can provide a high quality thermal oxide film.
After investigating the above problems in the thermal oxide film, we have found that they are caused by a reduced purity of nitrogen (N2) atmosphere gas during the cooling or the removing step and can be solved by blending a small amount of oxygen O2 in the high purity N2 atmosphere gas while eliminating impurities in the air during these steps, resulting in completion of this invention.
This invention, which can achieve the above objectives, provides a process for manufacturing a semiconductor device comprising heating a semiconductor wafer in a diffusion furnace equipped with a load-lock chamber, characterized in that an atmosphere gas for the semiconductor wafer during removing the wafer from the diffusion furnace is a mixture of nitrogen gas and oxygen gas in such a level that the thickness of an oxide film on the semiconductor wafer is substantially constant.
In the process, after heating, the diffusion furnace is cooled to a predetermined internal temperature prior to removing the semiconductor wafer from the diffusion furnace, and during the cooling step, the atmosphere gas in the diffusion furnace is also the gas mixture.
Furthermore, during the semiconductor wafer is in the diffusion furnace, the load-lock chamber is evacuated before nitrogen gas is fed, and then the semiconductor wafer is transferred to the load-lock chamber while feeding the gas mixture to the load-lock chamber.
The process further comprises transferring the semiconductor wafer from the load-lock chamber to the diffusion furnace, and the above gas mixture is also used as an atmosphere gas for the semiconductor wafer during the transferring step and a preliminary processing in the diffusion furnace before initiating the heating step. The preliminary step comprises warming the diffusion furnace to a predetermined internal temperature and stabilizing the internal temperature of the diffusion furnace after the warming.
The gas mixture contains 0.1 to 10% of oxygen gas.
Nitrogen and oxygen gases used preferably have a higher purity of about 99.999% or more.